ИСТИНА

Gastrulation appears to be one of the main stages in the development of all living creatures, which have at least two germ layers. It’s a complex process of morphogenetic rearrangements, accompanied by cells proliferation, differentiation and direct movement. However, we don’t know much about the participation of low molecular weight regulators, such as monoamines, at this stage of development, especially in gastropods. Among monoamines, serotonin (5-HT) is known as a substance with a wide variety of functions and tightly associated with the nervous system. However, serotonin appears as early as the zygote stage in both vertebrates and invertebrates and located within cytoplasm and cell nuclei in cleaved blastomeres (Ivashkin et al., 2019). We found that increased serotonin levels during the cleavage stage of Lymnaea stagnalis development resulted in irreversible malformation at the gastrula stage (Ivashkin et al., 2015). Here we present a detailed description of cell movements during Lymnaea gastrulation in normal conditions and under a high serotonin level. Embryos were incubated in 10 mM immediate serotonin precursor (5-HTP) during 24 h post-fertilization to increase the intracellular 5-HT level. Blastula and early gastrula stages were stained for serotonin and tubulin. Cells shape and position were determined using phalloidin and nuclei staining. Results were analyzed using confocal microscopy with 3D reconstruction. Incubation in 5-HTP leads to an increased 5-HT level in all cells of early developing Lymnaea embryo. Under such conditions, we observed changes in the number of cell contacts in identified blastomeres, their size and shape high variations and delay in proliferation rate. In addition, there was no actin-dependent contraction at the apical pole of the cell on the vegetative pole of the embryo, where the blastopore will form. Such 5-HT-induced changes led to abnormal movements of the cell layers during gastrulation and the subsequent formation of exogastrula. Our results show that intracellular serotonin, as a neurotransmitter, is taking part not only in cell communication, regulated by the nervous system. It can also participate in intercellular regulation during the early stages of embryo development when differentiation of first neurons is only planned. It’s clear now, that serotonin plays a big role in a cell layer movement, but it turns out, that 5-HT might have a huge impact on differentiation, what we can assume based on our work.